Laser light has been used to initiate the ignition of fuel/oxidizer mixtures by use of laser-spark, air-breakdown ignition methods in which a single, high peak power laser light pulse from a Q-switched laser is used to initiate fuel ignition by generating high temperatures and an ionization plasma. These laser ignition methods and apparatuses are generally unreliable except within narrow ranges of fuel parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel and air temperatures.
After initial ignition, sustaining ignition of fuel/oxidizer mixtures is typically accomplished by use of a laser light pulse from a Q-switched laser with a pulse width and pulse energy which will provide the peak power density required to initiate plasma formation and to satisfy concurrently the need for time-averaged power for sustaining the ignition. This requires fragile, bulky laser excitation sources such as flashlamps or laser diodes which are often difficult to fit proximate to fuel combustion zones, particularly in fuel combustion zones in places such as aircraft engines. Fuel combustion zones are usually harsh, mechanically adverse environments that necessitate sturdy design or frequent replacement of ignition devices subjected to those environments.
Thus there is still a need for a laser ignition process which can reliably ignite gaseous or aerosol fuel mixtures within a broad range of parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel temperatures as well as a need for means for ignition within small spaces under mechanically adverse conditions.
Therefore, it is an object of this invention to provide a reliable ignition method and apparatus.
It is another object of this invention to provide a method and apparatus for laser ignition of gaseous or aerosol fuel mixtures within a broad range of parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel temperatures.
It is yet another object of this invention to provide an economical method and apparatus for laser ignition of gaseous or aerosol fuel mixtures.
It is a further object of this invention to provide a method and apparatus for laser ignition of gaseous or aerosol fuel mixtures within small spaces under mechanically adverse conditions.
It is yet a further object of this invention to provide a method and apparatus for elimination of fragile and bulky laser excitation sources such as flashlamps or laser diodes from fuel igniting lasers located proximately to fuel combustion zones.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims which are intended to cover all changes and modifications within the spirit and scope thereof.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, there has been invented a fuel ignition apparatus and method in which an excitation light source is used to activate one or more small ignitor lasers located remotely from the excitation light source and more proximately to one or more fuel combustion zones. Using a separate excitation light source to pump the lasers which ignite the fuel eliminates the need for large, heavy, fragile or complex excitation sources in the harsh operating environments of fuel combustion chambers. Durable, reliable, economical ignition of gaseous or aerosol fuel mixtures can be accomplished with the invention apparatus and method.
In a first embodiment of the invention, a long duration low peak power pulse from an excitation light source is split and injected into at least two optical fibers. The first optical fiber transports the light into an ignitor laser. The second optical fiber is longer than the first optical fiber and transports a long duration low peak power pulse of the light into a beam combiner where, after the delay caused by the greater length of the second optical fiber, the long duration low peak power pulse is combined with the output of the ignitor laser. The output of the ignitor laser is a short duration high peak power pulse because the ignitor laser functions as a beam compressor. The combined beam of short duration high peak power pulses from the ignitor laser and long duration low peak power pulses from the fiber optic delay line is then focused into a focal point in an aerosol spray or cloud of combustible fuel.
In a second embodiment of the invention, a long duration low peak power pulse from a remote excitation light source is split and injected into at least two optical fibers, one of which is longer than the other. The first optical fiber and the second optical fiber both transport light into a beam combiner, with the portion of the light carried by the longer optical fiber being delayed in reaching the beam combiner by the greater distance it travels. A third optical fiber transports the combined beam into an ignitor laser which is activated by the combined beam. Because the ignitor laser functions as a laser pulse length compressor, the first output of the ignitor laser is a short duration high peak power pulse. Before the Q-switch on the ignitor laser has time to reset, a long duration low peak power pulse from the fiber optic delay line comes through the ignitor laser. The sequenced operation of the ignitor laser provides ignitor laser output of an alternating sequence of short duration high peak power pulses and long duration low peak power pulses. Output of the ignitor laser is focused into a focal point in an aerosol spray or cloud of combustible fuel.
In a third embodiment of the invention, light beams from a remote excitation light source are transported by an optical fiber to at least one ignitor laser which is positioned to direct a beam into a focal point in an aerosol spray or cloud of a combustible fuel. The excitation light source is used to provide a sequence of two low peak power pulses separated by a short time interval. After the ignitor laser is pumped by the first of the pulses from the excitation light source, the ignitor laser outputs a short duration high peak power pulse which breaks down and ignites the fuel. Before the ignitor laser Q-switch is reset, the second of the long duration low peak power pulses from the excitation light source re-energizes the ignitor laser to produce a long duration low peak power pulse in the output of the ignitor laser. The long duration low peak power pulse is also focused into the focal point in the aerosol spray or cloud of the combustible fuel to sustain the plasma formed by contact of the short duration high peak power pulse with the fuel.
In a fourth embodiment of the invention, excitation light is provided by a laser with at least two resonator cavities with a common high reflecting end mirror and separate output couplers. The sequenced pulses of light from the single excitation laser are transported by two optical fibers, one directly from the excitation laser to at least one ignitor laser and another directly from the excitation light source to a beam combiner where it is combined with the output from the ignitor laser. The combined beam of the sequenced pulses is focused into a focal point in an aerosol spray or cloud of a combustible fuel where the first short duration high peak power pulse from the ignitor laser breaks down and ignites the fuel and the second long duration low peak power pulse directly from the excitation laser sustains the plasma formed by contact of the short duration high peak power pulse with the fuel.
In a fifth embodiment of the invention, a light source capable of producing more than one wavelength of light is used as the excitation light source for activating a remote ignitor laser and sustaining a breakdown plasma in fuel. Two different wavelengths of light, the second of which is not within the absorption band of either the laser rod or Q-switch of the ignitor laser, are sequentially injected into a single optical fiber and transported to the remote ignitor laser. The pulse of light with the wavelength which is absorbable by the laser rod is compressed by the Q-switched ignitor laser to produce an ignitor laser output of a short duration high peak power pulse that is focused into a focal point in an aerosol spray or cloud of combustible fuel, thereby forming a breakdown plasma The subsequent pulse of light having the wavelength which is not absorbable by the laser rod or Q-switch passes unimpeded through the ignitor laser as a long duration low peak power beam is also focused into the focal point in the aerosol spray or cloud of the combustible fuel to sustain the plasma formed by contact of the short duration high peak power pulse with the fuel.
Any of the invention apparatuses can have an optical switching feature positioned to receive output of a single excitation light source and to direct beams in an ordered sequence or in a random sequence through beam splitters into pairs of first and second optical fibers, each pair of which is associated with an ignitor laser. When it is desired not to use a fiber optic delay line or a second optical fiber direct to the fuel combustion zone, as is the case of the third and fourth embodiments, the multiplexing feature is positioned to receive output of a single excitation light source and to direct beams in an ordered or random sequence into single optical fibers which are each connected to an ignitor laser.